There are several third-party installer applications out there: Stuffit, Mindvision, and others. For any standard OS X application, there is absolutely no reason to use these. Like rebooting the user's computer, using an installer is giving your user one of those two very bad messages. So do not do it.
Installers also suffer from another problem—the "Where did it go?" affliction. When an installer is done, you might receive a hint as to where the application was installed, but that is far from guaranteed. It is better to let the user decide where they want your application to live, rather than tell them where you want it to live.
Adobe and Microsoft are noticeable offenders in this area. Drag-and-drop is King on OS X. Use of third-party installers cries out "We do not care about your experience with our software."